Rework sidebar to use padding on the sidebar_content div

This saves every header, list, paragraph etc having to sort out their own padding. The couple of instances where we want edge-to-edge can be acheived using the negative margin spacing utilities.

The padding is based on $spacer so that it can be adjusted automatically via bootstrap configuration.

This also means that we can remove many (mis-)uses of the browse-section class, which is only supposed to be for cases where there are multiple browse-sections in sequence (e.g. multiple nodes in the node history browse pages).

app/views/application/_sidebar_header.html.erb

@@ -2,7 +2,7 @@
   <div class="flex-grow-1 text-break">
     <h2><%= title %></h2>
   </div>
-  <div class="px-3 py-3">
+  <div>
     <a class="geolink" href="<%= root_path %>">
       <span class="icon close"></span>
     </a>